Numerous devices are utilized for producing substantially circular holes in masonry, metal, and like material. One such device for drilling masonry is the "star drill" which is an elongated tool having a toughened star-shaped cutting end which is repeatedly driven against the masonry thereby causing the surface immediately below the cutting end to become pulverized. This pulverized material can then be removed from the hole in any suitable manner as by a stream of air. More commonly, however, a rotary drill bit is utilized for preparing such holes in masonry and metal. The most common of these rotary bits is a specialized form of twist drill utilizing a piece of hardened material, such as tungsten carbide, inserted transversely across the cutting end of the drill bit and this hardened material carries the cutting lips of the drill. This is referred to as a spade drill. Conventionally the spade drill bit has a pair of flutes extending the length of the drill to a point where a shank is formed for securing the bit in a rotary drive mechanism. The edges of the flutes (or lands) are normally dimensioned such that they do not contact the wall of a drill hole in order that a reduced friction between the drill and the wall hole is achieved. The flutes are utilized to withdraw cut material as the drill bit moves into the material being drilled. Because of the hardness of masonry (including rock, brick, etc.), and metal, the drilling operation is normally quite slow, and rather high pressure must be exerted axially on the drill bit to accomplish the drilling. Further, the cutting lips gradually become rounded requiring a resharpening of the bit. Part of this rounding occurs because the insert reaches substantially high temperatures due to friction against the masonry or metal material.
Although the cutting lips of a newly manufactured spade drill bit are normally symmetrical, the symmetry may be destroyed when the cutting lips are restored as by grinding with an appropriate sharpening apparatus because it is difficult to grind each of the lips equally. When asymmetry occurs, the drill bit tends to move from an axial cutting position and wander from the axis of the desired hole. Furthermore, the cutting lip which performs the greatest amount of cutting from this non-symmetrical shape is dulled more quickly and thus the drill bit requires more frequent sharpening.
Accordingly, it is one object of the present invention to provide a spade drill bit for rotary operation which penetrates masonry and metal material at a higher rate with less pressure, and thus develops less heat, than drill bits of the prior art.
It is another object of the present invention to provide a rotary spade drill bit which achieves cutting along a true axis of the desired hole in masonry or metal material.
It is also an object of the present invention to provide an improved spade drill bit which requires less sharpening maintenance.
It is still another object of the present invention to provide a rotary spade drill bit which is composed of two separate parts, the cutting bar and the shank, whereby the cutting bar can be selectively inserted and secured to the drill bit shank for use in selected drilling assignments.
Other objects and advantages of the invention will become apparent upon reading the detailed description with reference to the drawings.